[Someone]

Slight nitpick: higher MPG does not necessarily mean less emissions per mile; car engines do differ in the amount of emissions they produce per gallon of gas consumed.

For example, adding a catalytic converter to a car may decrease both MPG and emissions (http://en.wikipedia.org/wiki/Catalytic_converter#Environment...), so removing it may increase both MPG and emissions. And that is even disregarding the fact that adding that converter increases the weight of the car.

[zurn]

Also the fuel has a large impact on the MPG since fuels differ in energy content per volume.